Various strains of saprophytic soil microorganisms are known to influence plant growth in different types of plants. Increases in yield of agronomic crop plants (angiosperms), for example, can be obtained through inoculation with strains of selected soil bacteria. Among the bacteria that exhibit beneficial effects on crop plant development, perhaps the most promising are those which characteristically colonize the surface of the plant roots. As distinct from other plant-beneficial soil isolates that inhabit the area surrounding the root, the "root-colonizers" i.e. rhizobacteria, are able to transfer from seed inoculated therewith to roots developing from the seeds and are able to maintain a stable and growth-promoting association with the growing root system of the crop plant.
Since their discovery, members of the plant growth promoting rhizobacteria (PGPR) family have been shown to exhibit a wide range of beneficial effects on agronomic crops. One group of these root-colonizers is capable of exerting growth promoting effects directly on the plant; another group, the biocontrol PGPR, is capable of reducing the influence of pathogens in the root zone (see Kloepper et al, 1988, ISI Atlas of Science: Animal and Plant Sciences, pp. 60-64; and see Davison, 1988, Bio/Technology, 6, pp. 282-286). The plant growth promoting rhizobacteria have also been shown to exhibit beneficial effects on seedlings of fruit trees, such as apple trees (Ceasar and Burr, 1987, Phytopathology, 77, pp. 1583-1588) and citrus trees including rough lemon and sweet orange.
The possibility of employing beneficial microorganisms as inocula is also being researched by the forestry industry and, in particular, by the conifer industry which is responsible for supplying the vast numbers of conifer seedlings required annually for outplanting to reforestation sites. Conifer nurseries currently experience a seedling mortality rate of approximately 30% in the greenhouse, owing to poor germination, disease and other factors causing stunted seedling growth. Of those conifer seedlings grown successfully in the greenhouse, a further 18-50% either cannot survive or fail to establish after planting at the reforestation site.
Of particular interest to the conifer seedling industry are microbial agents capable of enhancing the rate at which the nursery-grown seedlings mature, and microbial agents that act, more specifically, to enhance development of the conifer seedling root and shoot system both in the greenhouse and during the critical growth stages which follow outplanting of those seedlings in the field. However, since microorganisms capable of promoting these beneficial effects on seedling growth must be capable also of tolerating the harsh i.e. cold and acidic, soils in which the seedlings are ultimately planted, the search for candidate microorganisms has been particularly demanding.
To identify microorganisms capable of surviving at the conifer root zone, Kampert et al (Polish Journal of Soil Science, 1975, 8(3):59-66) inoculated conifer seed and seedlings with forest soil and then studied the microbial flora resident in the seedling root zone. Their studies revealed that only a limited variety of microbial species contained in the soil inocula were capable of inhabiting the seedling rhizosphere. More particularly, they found that while the rhizosphere was colonized by various species of fungi such as Fusarium, Penicillium and Trichoderma, the colonizing bacteria were limited almost exclusively to those of the Coryneform group. Though the Coryneforms were capable of colonizing the conifer seedling root zone, subsequent studies revealed that inocula containing the Coryneforms caused a reduction, rather than an increase, in the length of pine seedling roots and shoots.
Ectomyccorhizzae species capable of inhabiting the conifer root zone, such as the Fusarium sp., are currently showing some promise as inocula beneficial to conifer seedling growth. While the mechanism through which the ectomycorrhizae offer these advantages is not well understood, it is believed that the ectomycorrhizal mantle (a sheath of ectomycorrhizal hyphae) that forms around the conifer roots serves to protect the root from pathogens and/or provides for enlargement of the root mass and increased nutrient uptake by the plant. While the use of ectomyccorhizal inocula appears attractive, difficulties with large scale production of these organisms has limited their commerical use.
Given the pressing need in the forestry industry for inoculants capable of enhancing seedling growth and survival on reforestation sites, it is clear that additional strategies for promoting conifer seedling development would be of considerable benefit.
It is an object of the present invention to provide a method for enhancing the growth of conifer seedlings.
It is a another object of the present invention to provide an inoculant composition suitable for promoting the growth of conifer seedlings.
It is a further object of the present invention to provide conifer seed on which has been applied a composition capable of promoting the growth of conifer seedlings.